1. Field of the Invention
The present invention relates in general to a coupling structure for coupling metal pipes, and more particularly to a coupling structure for coupling two metal pipes without using welding.
2. Description of the Prior Art
In the field of motor vehicles, various attempts have been made for reducing the weight and cost of the engine system and thus those of the vehicle. One of them is to reduce the thickness of metal pipes which constitute an exhaust passage incorporated in the exhaust system of the engine. Hitherto, in assembling the exhaust system, arc welding has been widely employed for coupling the pipes. However, as is known, with reduction in thickness of the pipes, such arc welding becomes very difficult because it tends to produce welding defects, such as openings formed in the welded portion. Thus, recently, other coupling techniques without depending on the welding have been eagerly desired for the pipe coupling in the exhaust system. One of them is disclosed in Japanese Laid-open Utility Model Application 5-64595, in which a collar unit, a fastening band unit and fastening bolts and nuts are used.
For clarifying the invention, the coupling technique of the publication will be briefly described with reference to FIGS. 3 and 4 of the accompanying drawings. FIG. 3 is a partially sectional side view of a coupling structure through which two metal pipes are coupled, and FIG. 4 is a section view taken along the line IV--IV of FIG. 3.
In FIGS. 3 and 4, denoted by numerals 11 and 13, are first and second metal pipes which are to be connected in tandem. For the coupling, the pipes 11 and 13 have diametrically enlarged end portions 11a and 13a which are mated at their leading ends.
Within the mated end portions 11a and 13a of the pipes 11 and 13, there is concentrically disposed a collar unit 15. The collar unit 15 comprises an inner collar member 15a and an outer collar member 15b which are welded to each other at their axial end portions. At an axially middle portion of the outer collar member 15b is formed therearound an annular ridge 15b' which is in intimate contact with inner surfaces of the enlarged end portions 11a and 13a of the pipes 11 and 13, as shown.
Around the enlarge end portions 11a and 13a of the pipes 11 and 13, there is intimately disposed a fastening band unit 17 which, as is seen from FIG. 4, comprises upper and lower semicircular bands 17a and 17b which are fastened to each other through two pairs of bolts 19 and nuts 21. Each band 17a or 17b has a concave inner surface intimately contacting an outer surface of the enlarged end portion 11a or 13a of the pipe 11 or 13.
For holding the bolts 19 and nuts 21, each band 17a or 17b is formed at diametrically opposed ends with flanges (no numerals). As is understood from FIG. 3, when the bolts 19 are turned in a tightening direction, the two semicircular bands 17a and 17b are gradually pressed against the enlarged end portions 11a and 13a of the pipes 11 and 12. During this pressing, the enlarged end portions 11a and 13a are gradually pressed against the outer collar member 15b of the collar unit 15. Thus, finally, the two metal pipes 11 and 13 are tightly coupled having the enlarged end portions 11a and 13a thereof firmly sandwiched between the collar unit 15 and the fastening band unit 17. With this tight coupling, hermetic sealing is achieved between the two pipes 11 and 13.
However, due to inherent construction, the above-mentioned conventional coupling structure has the following drawbacks.
First, the coupling structure needs numerous parts for the assemblage, which brings about complicated construction, troublesome and time consuming assembling work and thus increased cost.
Second, as is shown in FIG. 4, even in the tightly coupling condition, the coupling structure has inevitably certain clearances W1 and W2 between the mutually facing flanges of the semicircular bands 17a and 17b. As is known, such clearances W1 and W2 produce areas near the flanges where only insufficient fastening force is produced by the fastening band unit 17. In this case, undesired gas leakage through such areas tends to occur.